High affinity, stereospecific recognition sites (receptors) for neurotransmitters, neuromodulators, and many clinically useful drugs have been identified in both peripheral tissues and the central nervous system. The interaction of a neurotransmitter, neuromodulator or drug with a specific recognition site initiates a series of events (for example, the opening of an ion channel or activation of an enzyme) resulting in either a physiological/behavioral response (in the case of a neurotransmitter or neuromodulator) or pharmacological effect in the case of a drug. Furthermore, the presence of recognition sites for synthetic compounds suggests that endogenous substances may also be present that mimic (or antagonize) the effects of exogenously applied substances. Studies are in progress to characterize 'recognition-effector" systems, and to link novel recognition sites to effector systems under study include: a) the benzodiazepine/GABA receptor chloride ionophore complex; b) the glycine-gated chloride ionophore; c) "peripheral-type benzodiazepine receptors (in both peripheral tissues and the central nervous system); d) receptors for central stimulants (e.g. amphetamine, methylphenidate); e) recognition sites for hallucinogens (phencyclidine), and f) recognition sites for compounds that regulate voltage - sensitive calcium channels.